This isn't strictly a tube issue, but oh well. I have a tube linestage where there is increasing hum as the pot is increased until it is all the way up, at which point it is totally silent. That is, quiet at the volume extremes, and increasing noise as it is increased. Does sound indicative of something in particular?
For more info, this is actually a DAC that uses a TDA1541 and a resistor I/V which feeds a 6dj8's grid directly. I have tried to add a pot between the DAC chip and the gain stage, and am having this noise. The tube stage is a CCS loaded grounded grid.
For more info, this is actually a DAC that uses a TDA1541 and a resistor I/V which feeds a 6dj8's grid directly. I have tried to add a pot between the DAC chip and the gain stage, and am having this noise. The tube stage is a CCS loaded grounded grid.
Have you tried a 10K grid stopper directly on the tube socket ?
The 6DJ8 (a vhf tube) is notorious for oscillating at will and won't tolerate lanky wiring.
The fact that the pot control effects the hum, is probably a symptom of the tube oscillating in the RF trying to modulate any noise pickup and the pot extremes dampen oscillation conditions due to reduced Z. This used to be a common 1960's problem with a duff wiper.
A scope won't pickup this problem, the probe capacitance can be a hoax just enough to dampen oscillations, but the best way is to measure cathode current, which will rise when tube is oscillating or behaving peculiar. A screening can may help.
Becareful when using a tube this condition and feeding a power amp... some very stange noises and perhaps radio signals can be heard..
richj
The 6DJ8 (a vhf tube) is notorious for oscillating at will and won't tolerate lanky wiring.
The fact that the pot control effects the hum, is probably a symptom of the tube oscillating in the RF trying to modulate any noise pickup and the pot extremes dampen oscillation conditions due to reduced Z. This used to be a common 1960's problem with a duff wiper.
A scope won't pickup this problem, the probe capacitance can be a hoax just enough to dampen oscillations, but the best way is to measure cathode current, which will rise when tube is oscillating or behaving peculiar. A screening can may help.
Becareful when using a tube this condition and feeding a power amp... some very stange noises and perhaps radio signals can be heard..
richj
From what you say, the pot may be picking up EMI from somewhere. It would be worst at the halfway point (in terms of resistance - this is probably an audio taper) of the wiper, because that's the point at which impedance from either end of the track is maximum. Screen the pot body - connect it to ground if you can. Shield the leads to the pot too and ensure that no AC-bearing wires are nearby. If their proximity to the grid can't be avoided, such as heater wires, twist then tightly.
Sounds like a grounding issue.
As mentioned the pot body should be grounded.
Also, do you have more than 1 piece of equipment in your system that is earth grounded? For example, if your line stage and the power amp your connecting it to are both earthed, it can cause problems.
I've had that problem using my PC as a source (which is earthed) to my amplifier (which is also earthed).
ray_moth said:
Tha sounds like it could be right. I increased the stoppers to 8K5, and it may have helped a little, but not much. I then found some coax wire and used it to make shielded connections from the dac board to the pot. One side was considerably quieter, but the other was not really effected -- not sure why that would be. And, while it was quieter, it still wasn't really quiet enough.
The pot body is grounded and there is no AC in the area though there is a regulated DC heater supply that goes right by.
I am thinking that since I have a 12V regulated supply that can supply a few hundred mA, but which is only supplying about 40 right now, that I may just put the pot after the gain stage and build a little mosfet follower. Not my ideal solution as I hate adding stages, but perhaps a necessary one.
This DAC is an old breadboard version that I was going to send to a friend without a preamp to play with -- hence the need for a volume control. Here is a pic, and the pot is where the spdif input is on this version, with the spdif being moved to the middle now -- I should add that grounding has all been redone since these pics:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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